In April 1911 Einstein was appointed as a full professor of theoretical physics at the German part of Prague's Charles University. By that time, he had already won acclaim as the author of his special theory of relativity and a number of successful studies in thermodynamics and molecular physics and in particular in quantum theory and statistical physics.
Einstein moved from Zurich to Prague together with his first wife, Mileva, and their two young sons, Hans Albert and Eduard. The Einsteins lived in quarter Smichov which was not considered the smartest part of the city, but their flat (now in Lesnicka Street No. 7) was modern (in contrast with Zurich there was already electricity installed) and Einstein liked to recall his walks to the Physical Institute in Vinicna Street over the Vltava river. Also, in Einstein's words, the Institute was "excellent with a beautiful library".
Although Eisntein did not establish a very close rapport with Prague, he admired the romantic historical city. He associated himself with a group of Jewish intellectuals who gathered in the evenings at Berta Fanta's home where philospohy was discussed and music played. Here he met Hugo Bergmann, Max Brod and Franz Kafka. While in Prague, Einstein was invited to the first legendary Solvay congress where he met Planck, Lorentz, Madame Curie, Poincaré, most of them for the first time. Scientists also came to Prague to visit Einstein. One of the most important was Paul Ehrenfest.
As a full professor, Einstein gave regular lectures on mechanics, molecular physics, and thermodynamics. In 1911 his lectures were held in the Clementinum, in 1912 in the building of the present Faculty of Natural Sciences in Vinicna Street.
While in Prague, Einstein's interest in quantum theory diminished and his systematic work on a relativistic theory of gravitation began. In Prague he published eleven papers, six of them concerned with the theory of relativity (the non-relativistic works are devoted to the theory of specific heats and interaction of radiation with matter). In fact, his papers published during and shortly after his Prague period are particularly significant in that they paved the way to his general theory of relativity.
This effort was briefly summarized by Einstein himself in his foreword to the Czech edition of 1923 of his famous little popular book "About the Special and General Theory of Relativity in Plain Terms":
"I am pleased that this small book ... should now appear in the native language of the country in which I found the necessary concentration for developing the basic idea of the general theory of relativity which I had already conceived in 1908. In the quiet rooms of the Institute of Theoretical Physics of Prague's German University in Vinicna Street, I discovered that the principle of equivalence implies the deflection of light rays near the Sun by an observable amount ... In Prague I also discovered the shift of spectral lines towards the red ... However, the decisive idea of the analogy between the mathematical formulation of the theory and the Gaussian theory of surfaces came to me only in 1912 after my return to Zurich, without being aware at that time of the work of Riemann, Ricci, and Levi-Civita. This was first brought to my attention by my friend Grossmann ..."
The foreword clearly emphasises two important effects which Einstein discovered in Prague: the deflection of light and the gravitational redshift (in fact, the redshift had already been discussed in Einstein's paper of 1907, in the foreward wrongly dated as 1908). The first effect was described in the famous paper of 1911 "On the influence of Gravity on the Propagation of Light". However, the paper is of even greater importance: it contains what Eddington considered to be the original statement of the principle of equivalence, one of the most remarkable ideas in the history of science.
Also, the foreword does not mention Einstein's remarkable advance in understanding the basic ideas and features of a complete relativistic theory of gravity. These can be found in his other relativity papers from Prague. They include the equations of motion of a particle in a given gravitational field (derived from a variational principle), the influence of a given gravitational field on other physical systems (such as the electromagnetic field), or the non-linear character of field equations which describe the gravitational field due to a given source. All these necessary ingredients of a relativistic gravitation theory Einstein arrived at during his Prague stay.
However, during this period Einstein still assumed that the gravitational field can be described by a single function. This assumption lead to insurmountable difficulties and prevented him achieving the final formulation of the general theory of relativity during his time in Prague.
Albert Einstein left the city of Prague after his sixteen-month long stay in July 1912 when he accepted the chair of theoretical physics at the Polytechnical Institute of Zurich. Direct evidence suggests that Einstein was happy during his professorship in Prague.
web page J.Podolsky, 30 Dec 1997,